Multiple-Input Multiple-Output (MIMO) refers to a method of using multiple transmission antennas and multiple reception antennas to improve data transmission/reception efficiency. Namely, a plurality of antennas is used in a transmitter or a receiver of a wireless communication system so that capacity can be increased and performance can be improved. Hereinafter, MIMO may indicate multiple antennas in this document.
MIMO technology does not depend on a single antenna path to receive a single total message. Instead, MIMO technology collects data fragments received via several antennas, merges the data fragments, and completes data. The use of MIMO technology can increase system coverage while improving a data transmission rate within a cell area of a specific size or guaranteeing a specific data transmission rate. MIMO technology can be widely used in mobile communication terminals and relay nodes. MIMO technology can overcome the limitations of the restricted amount of transmission data of single antenna based mobile communication systems.
The configuration of a general MIMO communication system is shown in FIG. 1. A transmitter is equipped with NT transmission antennas and a receiver is equipped with NR, reception antennas. If a plurality of antennas is used both in the transmitter and in the receiver, theoretical channel transmission capacity increases unlike the case where only either the transmitter or the receiver uses a plurality of antennas. Increase in channel transmission capacity is proportional to the number of antennas, thereby improving transmission rate and frequency efficiency. If a maximum transmission rate when using a signal antenna is Ro, a transmission rate when using multiple antennas can be theoretically increased by multiplication of the maximum transmission rate Ro by a rate increment Ri. The rate increment Ri is represented by the following equation 1 where Ri is the smaller of NT and NR.Ri=min(NT, NR)  [Equation 1]
For example, in a MIMO communication system using four transmission antennas and four reception antennas, it is possible to theoretically acquire a transmission rate four times that of a single antenna system. After an increase in the theoretical capacity of the MIMO system was first proved in the mid-1990s, various techniques for substantially improving the data transmission rate have been actively developed. Several of these techniques have already been incorporated into a variety of wireless communication standards including, for example, 3rd generation mobile communication and next-generation wireless local area networks.
Active research up to now related to the MIMO technology has focused upon a number of different aspects, including research into information theory related to MIMO communication capacity calculation in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of MIMO systems, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.
To describe a communication method in a MIMO system in detail, a mathematical model thereof is given below. As shown in FIG. 1, it is assumed that NT transmission antennas and NR reception antennas are present. In the case of a transmission signal, a maximum number of transmittable pieces of information is NT under the condition that NT transmission antennas are used, so that transmission information can be represented by a vector represented by the following equation 2:S=[S1, S2, . . . , SNT]T  [Equation 2]
Meanwhile, individual transmission information pieces S1, S2, . . , SNT may have different transmission powers. In this case, if the individual transmission powers are denoted by P1, P2, . . . , PNT, transmission information having an adjusted transmission power can be represented by a vector shown in the following equation 3:Ŝ=[Ŝ1, Ŝ2, . . . , ŜNT]T=[P1S1, P2S2, . . . , PNTSNT]T  [Equation 3]
In Equation 3, using a diagonal matrix P of a transmission power, Ŝ can be represented by the following equation 4:
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
A weight matrix W may be applied to the information vector Ŝ having an adjusted transmission power to configure NT transmission signals x1, x2, . . . , xNT to be actually transmitted. In this case, the weight matrix is adapted to properly distribute transmission information to individual antennas according to transmission channel situations. The transmission signals x1, x2, . . . , xNT can be represented by the following equation 5 using a vector x, where wij is a weight between the i-th transmission antenna and the j-th information, and W is a weight matrix or a preceding matrix.
                    x        =                              [                                                  ⁢                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      t                                                                                            ]                    =                                                    [                                                                                                    w                        11                                                                                                            w                        12                                                                                    …                                                                                      w                                                  1                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    …                                                                                      w                                                  2                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      w                                                  iN                          T                                                                                                                                                ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            …                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                j                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                  s                  ^                                            =              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
Meanwhile, in a spatial multiplexing scheme, the individual transmission information pieces Sl, S2, SNT have different values. In a transmit diversity scheme, however, the individual transmission information pieces S2, . . . , SNT have the same value. A general transmit diversity scheme includes Space Time Block Coding (STBC), Space Frequency Block Coding (SFBC) and Cyclic Delay Diversity (CDD).